Method of making chromium and its alloys



April 16, 1946'. T. A. MITCHELL 2 5 METHOD OF MAKING CHROMIUM AND ITS ALLOYS Filed Dec 19, 1959 v 2 sheets-sheet 1 Lininq of Reaqant materials Fused Bath 0; SNaF-Al F Coniaxnuyq AFZG3+CFZO3 4 ,4 2 ingot of Chromium and. p/souauua Baih Sfoak. 4'6 10 W 9 metal. Cathode.

inventor witness v (Itforneg 71611281? 8. Covey p 6- T. A. MITCHELL 9 METHOD OF MAKING CHROMIUM AND ITS ALLOYS Filed Dec. 19, 1959 2 Sheets-Sheet 2 C T' O I A1 03 F3 StOCk r aledrd ujc Cell Melhnq l Po: lnqoi of Chromium Meial+BcLih Stock Cr'gsh Grind H O fClasSihJinq Tcpbl H O Metal Conce dr'cdes- Bach Shock Bach Sifock & Drger" Dryer Melhnq Furnace Volahhzed Chromium A1303 Mecql Slag 3nventor Thomas Amv hen Z'Jiiness (Ittorneg Herberi $.Coveq ctenteci Apr. '16, 1946 METHOD or MAKING CHROMIUM AND TS Thomas A'. Mitchell,

ALLoYs Torrance, Calif.', assignonby mesne assignments, to Molybdenum Corporation of America, New York, N. Y., a corporation of Delaware Application December 19,1939, Serial No. 310,009

4 Claims.

This invention relates to an electrochemical method of making chromium and its alloys'and more particularly to the production of such metals which have a very low content of carbon and other undesired impurities.

For various industrial purposes, such as in the production of nickel-chromium-iron electrical resistor elements aswell as various other chromium but theproduct containssome aluminum in spite of careful control of the reaction. Chromium may also be obtained by reduction of the oxide by means of carbon. Also a term-chromium alloy may be made byre'duction of chromite by means of coke and a suitable flux. These reactions, however, do not result in a pure product. For example, the carbon content of ferro-chromium produced by smelting chromite mayvary from 4 to 8%; and to obtain a low carbon alloy it is necessary to refine the high carbon product i of the'first stage, andgthis operation must be carefully controlled to prevent loss of the chromium metal. Other commercial processes present similar problems, wherein the operation is either expensive or does not result in a pure or satisfactory product for many desired uses.

The primary object of this invention is, therefore, to overcome such problems and to produce chromium which issubstantially free from carbon and other undesired impurities.

A further object is to provide an electrochemi- .1

cal method for producing chromium metal and alloys thereof which. is easily controlled and eco nomically carried on and will give a product of a desired composition.

Another object isto provide a method of obtaining substantially pure chromium or an alloy thereof from impure rawmaterials. Other objects will be apparent in the following disclosure.

In accordance with this invention, I propose to make metallic chromium by the electro-deposition of magnesium or aluminum metal from a suitable fuscd'bath in the presence of chromium oxide or other suitable compound, whereby the reagent metal. aluminumor magnesium, reacts as it isiormed to displace the chromium from its compound and produce chromium metal. The

electrolyte is so constituted that itwill dissolve compounds. ofthe reagent metal and chromium, and an excess or the latter ismaintained. in the fused bath to insurereaction withall of, the reagent metaldeposited or tending to deposit at the cathode. Also, the bath constituents are 1 preferably so selected'and the temperature of the furnacegoperation so maintained that the reagentmetal is deposited in 'a fluid condition at the cathode while the chromium-orits alloys are produced in a solid'non-moltencondition as aspongeQtree, pelletsyingot, or in other form. 1 The alloys of chromium withother metals, such as Fe, Ni and Cu, may be made by including one or more of the oxides of the desiredJ-alloying metals in the cell bath, and these will be reduced by the reagent metal along with the chromium. The mixture "of deposited metal and solidified bath stock maybe separatednnd the metal purifled by further procedure comprising concentration of the metaland melting.the concentrates as will be explained. i In theaccompanying' drawings: i Fig. 1 is a diagrammatic vertical section of an electrolytic cell; and I Fig. 2 is a-fiow diagram of theprocess.

The nature of the electrolyte andthe constructional features of the cell aregoverned by the requirements for producing the reagent metal aluminum or magnesium under the above specii fled conditions. The electrolyte may" be any suitable 'fusedfsubstance capable of dissolving chromium oxide as well asserving for the electrolytic production of the'r'eagent metal, such as thefluoride of aluminum or magnesium used in suitable'proportions with the fluoride of one or more metals electropositive thereto,- and particularly the-alkalinemetals, sodium, potassium and calcium. Aluminum is molten at 660 C.

and'magnesium at 651 C., while chromium melts at 1615 C. The'bath constituents are. therefore, so selected and the furnace cell is sooperated that the bathtemperature is betweenthe melting points of chromium and the reagent metal, or above 660 C. and below 1615 C. A satisfactory electrolyte may comprise both alu minumand' an alkaline metal combined with fluorine, and preferably cryolite, sodium aluminum fluoride which melts at about 1000 c. and

is a solventfor alumina, magnesia and,chr omium oxide. This fused bath may contain from 5 to 10% of aluminaor magnesia as well as an excess of chromiumoxide suspended ordissolved therein;.and whensubjected to a direct current electrolysis, molten aluminum or magnesium in the process.

metal tends to deposit at the cathode and there reacts in a nascent condition with the chromium oxide or other compound in solution and thus serves to build up a deposit of the metal chromium on the cathode plate.

. The electrodes are so constituted and the process is so carried on as to minimize contamination of the product with carbon. Molten chromium will dissolve carbon readily; hence it is primarily desirable that the chromium product be held at a temperature below its melting point and that carbon be absent in the cathode zone. The starting cathode is a non-carbon member, such as a plate of chromium, iron or other substance. After the process has been started, the

The

chromium deposit serves as the cathode. anode may be made of suitable material, such as carbon, and the cell will be so operated as to maintain the anode at a distance well out of contact with the chromium deposit as it builds up.

If magnesium is used as the reagent metal in this process, magnesia may be dissolved in sodium .fluoride or cryolite. The electrolysis of this bath without the oxide of chromium would result in the production of molten magnesium which would tend to float'to the top of the bath and there burn when contacting with oxygen. However, the presence of chromium oxide orpther. compound capable of reacting instantaneously with .the magnesium molecules as they are formed at the oath-' ode reconveits that magnesium metal to the oxide where it again returns into solution for recycling Aluminum metal is similarly formed by electrolysis of, the dissolved alumina, and the reaction with the chromium oxide reforms alumina which goes into-solution in the bath.

The chromium oxide to be reduced may be fed as a dry powder onto the top of the fused bath, or it may be supplied to the bath as a cell lining, 'or preferably both methods are adopted. Care should, however, be taken to avoid contact of the chromium oxide with the carbon anode, since there may be a direct reduction of the oxide and the formation of chromium carbide. In order to form a satisfactor heat insulation for the cell walls and avoid detrimental reactions between the fused bath and the container wall, I may line the furnace with chromium oxide or a mixture of chromium oxide and the solid bath constituents. These may be used in such proportions that the bath will be replenished therefrom without change in proportions or'concentration as the cell is operated. One may also introduce further charge material into the top of the furnace, and particularly to maintain a cover of undissolved and unmelted material to limit the oxidation of the anode. The chromium oxide will dissolve from this cover layer and from the lining and thus maintain a uniform concentration of chromium reagent in the bath, which is high and approaches saturation because of the continuous contact of 'undissolved chromium oxide with the solvent bath.

The process may be carried out'in any suitable apparatus adapted for a fused bath electrolysis. This cell is preferably so made that the electrodes may be separated as the deposit is built up; and for this purpose I may so arrange the structure that the cathode and its contained cell may be moved downwardly away from an adjustable but normally stationary anode. The construction-may be as shown and claimed in my copendingapplication Serial No. 250,396 filed January 11. 1939, and which is diagrammatically on the cathode plate.

sections for the sake of rings are built up one above the other to form illustrated in the accompanying drawings as a vertical section of the cell.

The construction illustrated comprises a set of U-shaped steel rings l0 which may be made in replaceability, and these copper or other suitable non-carbon material which forms the cathode. Suitable insulation 20 separates this cathode from the metal structure forming the cell chamber. Mounted above the cathode, is an anode 22 made of suitable material, such as carbon. This anode may be suitably suspended by means of a clamping rin 24 and other adjustment mechanism arranged for raising and lowering the same as required. One may use either a multiple a'node.made of small cylindrical graphite rods or one may use a single large graphite body,,as shown in the drawings. Power may be applied to the cell by means of a clamping ring 26 to which is attached an electrical conductor for delivering a direct current thereto. An electrical connection ma be made to the cathode 20 by means of a roller 28 suitably mounted to ride against the upturned peripheral flange of the cathode plate which projects outwardly beyond the cell wall.

The fused bath may be stirred, if desired, by various types of mechanism and arrangement of the parts, such as a rotating anode or cathode. For

example, I may support the metal base l2.of the cell on a lower platform 30 provided with a set of rollers 3| suitably mounted thereonand hearing against a track 32 on the underside of the plate 12. A pivot pin 33 is axially mounted in a boss on the underside of the plate l2 and supported in a boss 34 forming the center portion of the platform 30, so that the cell bottom 12 will be rotatably guided by the pm. .A rin gear 35 on the underside of the plate I2 is driven by means of a gear wheel 36 rotated by any suitable source of power or by hand mechanism.

The electrolytic and chemical reactions result in the building up of a mass of chromium metal To prevent short circuiting by'contact of this deposit with the anode, I propose to move the two electrodes away from each other periodically or continuall duringthe process and thereby maintain proper electrolysis conditions within the cell. The anode may be made adjustable in position for this purpose as well as to feed it downwardly as oxidation consumes the rod. I prefer, however, to adjust the anode only to compensate for its being consumed and to move the cathode as the ingot builds up. This is preferably done by moving the entire cell downwardly relative to the anode. The cell including the metal cathode plate I8 and the solid deposit or ingot carried thereby may be lowered by means of a screw mechanism indicated diagrammatically by the screw 38, which is suitably mounted so that it may be operated by means of amotor or hand mechanism.

It is desirable that the chromium oxide or other reagent compound employed be fed slowly to the cell as required by exhaustion of the chromium in the solution. This may be accomplished by lining the cell Wall with a material containing chromium oxide, and preferably with a mixture of this oxide with alumina or other reagent metal oxide employed and, if desired, with solid material designed. to replenish th solvent bath.

Hence, theU-shaped rings I0, which are filled with a suitable insulating material 42, such as,

magnesia or asbestos. are'provided with an in nerlining M of these desired reagents and bath ingredients. For this purpose, the U-shaped channels of the rings l may be closed with plates 46 which thus form the inner wall of the cell; and this wall is arranged to carry a lining of the solid reagent material suitably cemented together and compressed in position and, if desired, baked thereon. This lining may form the sole source of the reagents and/or bath material, or it may supplement thematerial fed' into the top of the cell.

As aspecific example to further illustrate the process, I may employ for the sourc of raw chromium a'technically pure chromic oxide containing 65 or 66% of chromium contaminated with small amounts of Fe, Si, etc. The fused constituents .Then" the concentrates to which bath may besodium-aluminum-fiuoride, orthe mineral cryolite, herein termed an alkaline metal fluoride, to which may be added if desired, the

fluorides of other metals such as aluminum, calcium and sodium for varying the melting point or for any other desired purpose. The electrolyte contains from 2 to 5% byweight of alumina dissolved therein, as well as a suitable amount solved from this lining, but the thickness of the lining is such that it remains intactto protect the steel shell of the cell; The molten cryolite or returned bath stock may be initially poured into the cell and alumina and chromium oxide added thereto. A suitable directcurrent is applied to the electrodes at a voltage which is ordinarily from 5 to 8 volts, and with an amperage dependingupon the capacity of the cell, such as 3,000 to 10,000 amperes.

The temperature of the cell may be suitably maintained as by lowering the cell and increasing the gap between the electrodes, and thus raising the voltage. However, the reactions of aluminum or magnesium with the chromium oxide and of the evolved oxygen with the carbon of the anode are exothermic and aid in maintaining the bath temperature. The melting point of the bath will depend on the bath composition; and

the mixtureof fluorides will be controlled, as is well understood, to give a desired melting point. There may be some slight chemical decomposition of the bath at the operating temperatures employed, resulting in a slight loss of fluorine, and this loss may be made up by the addition of a suitable fluoride, such as cryolite or sodium, aluminum, calcium or chromium fluorides.

The reagent metal is in a molten and presumably nascent condition and it reacts readily with the available chromium oxide and/0r chromium fluoride in th solvent bath at a bath temperature below the melting point of chromium. Whether or not the chromium is momentarily molten as a result of the exothermic reaction of the alu minumand chromium oxide, it builds up as a arated from the metal.

solid tree, sponge, contacting granuiel; non-molten .form of igot. The bath is agitate by the evolving gas andthe magnetic field, which adds in dissolving the solid material as it is added. The [cell is continuously or periodically lowered to insure a. proper spacing between the anode and the chromium deposit which grows on the cathodei- As thecell islowered, the bath stock intermingled with the chromium tree-like deposit solidifies, sothat the ingot is made up of an intermixture of the bath-stock and chromium .metal. An analysis 'of the product of a typical furnace run is as follows: Cr--36%. (30% metallic) Al10%,-Na. -1 8%.and'F 28%." v

The impurities in the metal product, such as Si andfnl, may be eliminated along with the cell bath stock ingredients during amelting operation. Sucha procedure mayinvolve crushing the product and concentrating it by water classifica- .tion, whereby the high density chromium metal is largely" separated fromthelow density bath are attached some of the solidified bath are,

meltedin a suitable furnace which has-its walls made of such material as will avoid contaminanon of thefproduct. .In' this furnace, the bath constituents comprise principally cryolite containing dissolved. alumina or magnesia andan excess ofchromium oxideand/or fluorides. The

melting operation thus separatesth metal from the low densityjingredients, which may be suitably removed. Any silicon compo nd that is present as. arr-impurity will be converted tosilicon fluoride and, the temperature of the bath at which chro i mis molten is such that the silicon fluoride willbe volatilized and thus sepv This conversion of silicon to the fluoride is caused by providing a reactive metal fluoride, such as the fluorides" of alkali metal, aluminum, magnesium and chromium, and ordinarily therewill .be enough of the reagent in the entrapped bath stock to eliminate the silicon; but, otherwise, more of the reactive fluoride may be added for the purpose. Also, any aluminum or magnesium metalthat is present as a free metal or an alloy may be removed .by insuring the presence of sufiicient chromium oxide ,or fluorideso as to convert the aluminum to the oxide or'fluoride which separates from the metal as a low density slag. Magnesium is similarly separated into the slag and separated from the chromium metal. The

excess of. chromium oxide in the entrapped bath stock may be suificient for the purpose oi 'removing the reagent metal.

The accompanying flow sheet of Fig. 2shows the preferred procedure for converting impure or pure chromium'oxide into chromium metal.

As there illustrated. the cell may be initially started by the use of icryolite, alumina and chromium oxide. fThen when thecell run hasbeen finished, thefingot stock comprising' chromium metal with the entrapped solidified bath constituents is crushed "and thengroundto a fine size, such as will pass through a screenhaving 60 meshes per linear: inch. This material "is then concentrated on a standard Wilfieywater classification table or by other suitable apparatus. The

tailings from this tablef consist chiefly of the bath constituents, but with different proportions. These maybe dried and then returned for use in making up the bath for a second furnace run, by

being intermixed with calculated amounts or cryolite, alumina-and chromium oxide as is required for the electrolyte composition. The concenthe carbon reduction of that in factthe reactionsare carried on in the suitable type of melting pot, such as an induction furnaca-wherewthe above-described purification ofatheproduct is obtained. After separa- :tion of: theslag fromi the metal, the mass is cooled and theresultingingot is substantially pure chromium-metal, depending upon the operation of the=furnace andthe natureof the reagents em- 'metalsf areintroducedinto the fused bath. If iron, silicon onother' metal oxide is present either by intentional;introduction:or as an impurity in the chromium oxide orbathstock, the metallic aluminum or-magnesium'willreduce that oxide 2,398,591 trates on the WiIfiey table are separated from the I claim:' 1. The method of making chromium comprising the steps of electrolyzing in a cell between at inthe'fus ed bathalong with the'reduction of the chromium oxide and thus cause both metals to appearat :the cathode. The melting point of each. of these v metals is abovethat of the cryoline -bath, hence eachwill' be deposited at the cathode I ina solid ornOn-fluid condition when a fused bath of the abovedescribedcharacteristics is employed; Anymetalwhose melting point is above that of the selected bath and whose oxide is soluble in the fused bath and'iscapable of being reduced by the aluminum or magnesium may be formed asanalloy with the" chromium as above indicated. Hence, the claims pertaining. to the formation of chromium metal are to be interpreted as. coveringthe production of a chromium alloy where one; or more other metals are formed simultaneouslytherewith.

IUWlllenOW be'ap'preciated, in view of the above explanation, t-hat'this process does not involve chromium oxide, and

cathode zone away from carbon. This comprises essentially the production of a molten reagent metal by' electro-chemical deposition which in turn reacts with a chromium compound in the electrolyticbath, and this reaction causes chromium tobethrown out as a metal. This process does not involve the direct electro-deposition of chromium from a. chromium salt bath, but the metal is formed-by-reactionof a chromium compound, such as the oxide, with a more electropositivereagent metal which is formed electrolytically in. the presence of the chromium compound. It may also be observedthat the sodium of the cryolite or other metal ion present which is electropositive-tjo chromium'will remain in the bath, and the electrolytic action will r esult only in thedeposition ofthe reagent metal aluminum .or magnesium withanevolution of oxygen at the anode. Hence, the fluoride bath serves merely as-a solvent and-carrier for the reagent materials.

' It will now beappreciated that various modifications may be made in the process and that the compositions-andthe finalprodu'cts may be widely varied; hence, the above statements are to be considered as descriptive of the general principles of the process as well as a specific example thereof, and notes limitations on the claims appended thereto. While I have endeavored to explain the theories-which are believed .to underlie this process. the claimsarenot to be. interpreted as limited to any-theoryv orprinciple of: operation, since it is immaterial whether or. not thereagent metal is initially depositedelectrolytically at the cathode or ;how it aids informing the chromium deposit".

V bined with aluminum and a high concentration thereof during the anode and a metal cathode therebeneath a fused bath comprising a substantial amount of an oxide of a reagent metal selected from the group consisting of aluminum and magnesium dissolved in a solvent composed chiefly of a substance containing fluorine combined with aluminum and a metal selected from the group consisting of potassium, sodium and calcium, holding the bath fused at a temperature-below the melting point of chromium, progressively dissolving an excess of chromium oxide in. the bath and maintaining lectrolysis, progressively separating the anodeland the associated fused bathzonefrom said cat ode and the cell bottom and gradually forming on the cell bottom an ingot of solidified bath stock containing chromium metal which is substantially free from the reagent metal. a

2. The method of making chromium comprising the steps of .electrolyzing in a cell between an anode and a metal cathode located therebeneath'a fused bath containing about 2 to 5% by weight of alumina dissolved in a solvent composed chiefly of a substance containing fluorine coma metal selected from the group consisting of potassium, sodium and calcium, maintaining the bath fused at a temperature below-the melting point of chromium, contacting the bath with solid chromium oxide and progressively dissolving said oxide and maintaining during the electrolysis a high concentration thereof approaching saturation to insure that the ultimate metal product be substantially wholly chromium, progressively separating the anode and the fused bath from the cathode and the cell bottom therebeneath, and gradually building on the cell bottom an ingot of solidified bath stock containing chromium metal which is substantially free from aluminum metal.

3. The method of making chromium comprising the steps of electrolyzing in an open cell between an anode and a metal cathode therebeneath a fused bath comprising at least 2% of alumina dissolved in a solvent composed chiefly of cryolite, holding the bath fused at a temperature below the melting point of chromium, continuously contacting the bath with solid chr0- mium oxide and progressively feeding said oxide into the open top of the zone and maintaining during the electrolysis a highv concentration of dissolved chromium oxide approaching saturation to insure that the ultimate metal product be substantially wholly chromium, progressively moving the anode and the fused bath away from said cathode and the cell bottom and gradually building on the cell bottom an ingot of solidified bath stock containing chromiummetal substantially free from aluminum, and progressively supplying bath stock to maintain a predetermined volume of fused electrolyte inthe cell.

4. The method according to claim 1 in which the chromium metal product and its impurities are heated with the associated bath stock containing a fluoride and chromium oxide to a temperatureabove' the melting point of the chromium to complete the reactions and to purify the metal; after which the from the residue.

* THOMAS A. MITCHELL.

chromium is separated 

